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More and more sensors today provide intelligent remote parameterization and diagnostics that simplify handling and operation and lower total cost of ownership (TCO). The product group of these sensors of medium complexity is becoming increasingly important, but relies on a powerful efficient interface. Pepperl + Fuchs shows in this article how IO-Link comes into play here. It not only transmits all data via the same three-wire cable, but will also reduce the existing interface diversity to a few standards in the future.
Above: Sensor interface today
There are numerous historically developed standards for connecting sensors to control systems, many of which are still in use, but no longer do justice to the increased functionality of modern devices. A mix of various interfaces is therefore used in parallel in many applications. On the analog side, this includes the 4 ... 20 mA current interface and the 0 ... 10 V voltage interface while in the simplest case a switching output is sufficient for digital communication and otherwise serial point-to-point interfaces such as RS232 and Serve RS422. One level above are bus systems such as Profibus, CAN and the actuator sensor interface (AS-i) as well as the solutions based on Industrial Ethernet.
Multiple interfaces for the same sensor
The typical constellation in the area of sensors with medium communication requirements looks like this today: In order to enable access to the advanced functions of intelligent sensors, there is also one or more analog outputs or switching outputs for the actual sensor signals, for example an additional serial RS232 interface , One and the same sensor therefore requires several interfaces of different types. Such double and multiple connections cause unnecessary costs, which lie not only in the sensor and the control system, but also in the form of plug connectors, bus connections and sometimes complex shielded cabling.
From a technical and economic point of view, consolidation is long overdue, especially against the background of current innovations such as the trend towards “measuring instead of touching”. The latter is a new category of sensors that, in addition to the conventional switching signal, provide the exact distance to the detected target object and thus offer the user more protection against incorrect switching and greater process reliability. For the reasons mentioned, there will be no way around IO-Link in the future. As a serial digital point-to-point connection, the system is designed for such requirement scenarios. It is not only suitable for the transmission of analog and digital sensor signals, but also functions as a parameterization and diagnostic interface or other functions. IO-Link is consistently positioned in the lower area and serves almost the last mile from the sensor to the bus connection or, if necessary, directly to the control.
IO-Link: manufacturer-independent, inexpensive and flexible
Under no circumstances should IO-Link be understood as an additional bus system. The orientation and strengths of the system were not communicated clearly enough when it was first introduced a few years ago, so that misleading interpretations were circulating. In the meantime, a clear view has prevailed here and numerous products from various manufacturers are currently available with IO-Link. Almost all providers also offer corresponding input / output modules on the control side. Interestingly, the price of an IO-Link input channel is already slightly below that for an analog channel.
IO-Link is internationally standardized in the IEC 61131-9 standard, which ensures compatibility and investment protection regardless of the manufacturer. The interface supports transmission rates of 24 kbaud, 20 kbaud and 4,8 kbaud based on the 38,4 volt level and inexpensive unshielded three-wire cabling up to 230,4 m in length. A clear advantage over conventional analog interfaces is the unalterable data transmission, since analog measured values are already digitized in the sensor. IO-Link also offers enough reserves for time-critical tasks, the cycle times for the transmission of 16-bit values with the average speed of 38,4 Kbaud are 2 ms. Full downward compatibility with the PNP switching output is important for smooth migration.
Easy access using standard tools
With these key data, IO-Link is perfectly tailored to the requirement profile of sensors of medium complexity and allows complex rationalizations. In addition to the simple transfer of all conceivable data formats from the sensor to the controller and vice versa, there are a number of options for organization and administration. Each IO-Link sensor has a standardized device description IODD (IO Device Description), which users can easily access using suitable software tools. Configuration data of devices can be stored centrally in controls or databases and reloaded into the sensor without delay after product changes or an exchange. Cloning parameter sets is also a breeze.
The limits of IO-Link should not go unmentioned. The large field of binary sensors with a simple 24 V switching output remains largely unaffected by IO-Link. As a rule, it accounts for the largest number of sensors in the field of factory automation, which includes the many inductive and optical position switches. The transistor output according to PNP switching logic has established itself as the quasi-standard here, if one ignores some applications in the USA and Japan that still use the NPN variant. The use of IO-Link at this lowest sensor level would hardly offer any noteworthy advantages, but would rather make the connection more expensive. Above the area of application of IO-Link is the group of sophisticated sensors, which include, for example, vision sensors. As soon as large volumes of data are in focus, for example for the transmission of live images, Ethernet-based interfaces are the right choice as a communication standard.
The result: sensors with intelligent functions
IO-Link is the standardized, innovative and inexpensive sensor interface for sensors with intelligent functions. The serial digital system is suitable for the transmission of all communication data of a device, regardless of whether it concerns analog values, digital switching signals, or configuration and diagnostic data. So IO-Link plays a central role in reducing the unmanageable variety of interfaces to the three standards 24 V switching output, IO-Link and Ethernet. Since IO-Link offers full downward compatibility with binary switching outputs, users can implement IO-Link with practically no risk: IO-Link sensors work without any problems on a conventional digital control input, just like an existing sensor with PNP switching output on an IO-Link input can be connected.